Absorbent pads such as sanitary napkins which utilize cellulosic fiber as their principal absorbent, are well known. Cellulosic fiber provides a relatively inexpensive source of absorbent material but suffers from distinct disadvantages when utilized alone as an absorbent layer. One of the main disadvantages is that when a layer of cellulosic fiber is wet, it tends to collapse upon itself with the result that a saturated cellulosic fiber is dense, compacted, and relatively hard and uncomfortable.
This disadvantage has been recognized and thermoplastic fibers have been added to batts of cellulosic fibers in an attempt to retain the relatively inexpensive source of absorbency while introducing resistance to a permanent deformation and also resilience associated with the presence of these fibers. Representative examples of absorbent pads made with this combination of fibers can be found in U.S. Pat. Nos. 4,082,886; 4,129,132; 3,976,074; 4,054,141; 4,047,531; 3,545,441; 4,219,024; and 4,100,324. It has also been noted that when a layer of absorbent material containing thermoplastic fibers is folded upon itself, resiliency is increased even further in the sanitary product containing this folded absorbent layer.
There are difficulties inherent in using thermoplastic material in a cellulosic absorbent batt. The introduction of a thermoplastic material can decrease absorbent capacity and can also diminish the capillary attraction for the fluid to be absorbed. In order to counteract these problems, additional absorbent materials which absorb high levels of fluid per unit volume may be introduced underneath the absorbent layer or, in the case of a folded thermoplastic containing absorbent layer, the highly absorbent insert is positioned within the fold. An absorbent pad such as the one described above is described in more detail in U.S. patent application Ser. No. 266,795 filed May 26, 1981 which has been informally allowed and is hereby incorporated by reference. This particular application describes an absorbent insert made of surfactant treated meltblown microfiber.
When a sanitary napkin or the like is designed with a folded absorbent layer containing thermoplastic material and an absorbent insert is positioned within the fold, the precise positioning of the absorbent insert is important. The absorbent insert, e.g., wood pulp fluff, surfactant treated meltblown microfiber, superabsorbents, etc. performs its function of high fluid retention per unit volume due at least part to its relatively small capillary size compared to the cover. The small capillaries help to draw fluid through the upper layer and preferentially absorb fluid until the capacity of this layer is substantially utilized. If this absorbent insert was not positioned properly, or if the insert was repositioned during wear, the benefits derived from the presence of the insert would be minimized.
Recently, sanitary napkins have been designed with increasing bulk in the central napkin portion. One way of obtaining this increased bulk is by folding the absorbent layer so that an extra fold exists in the central portion of the napkin. This extra thickness, however, is difficult to maintain in the proper position.
Examples of patents describing folded absorbent layers in napkins are U.S. Pat. No. 3,667,468, 3,954,107, 3,699,966, 3,364,931. U.S. Pat. No. 3,183,909 is an example of a patent disclosing a sanitary napkin with a raised central layer.